This invention relates to a leadframe adapted to support semiconductor devices, and, more particularly, to a leadframe which enables ready cutting of outer lead sections thereof and which decreases portions of the outer lead sections to which the solder does not adhere when they are connected to the printed circuit board. This invention relates also to a method of mounting the leadframe on a printed circuit board.
Leadframes for supporting semiconductor devices thereon have outer lead sections, inner lead sections and a central die pad portion. Semiconductor chips are mounted on the die pad portion, and the pads of the semiconductor chips and the inner lead sections are bonded together through wires. A plastic package type semiconductor device is formed by molding in a resin the inner lead sections and the die pad portion, with the outer lead sections exposed. The outer ends of the outer lead sections are plated with a solder for mounting the outer lead sections on a printed circuit board.
In recent years, there has been a tendency to increase the number of input and output terminals of semiconductor chips having various sizes. Especially, in electronic devices, it is strongly desired to reduce the size and weight of the devices. For this reason, efforts have been made to reduce the size of the electronic devices and to provide a greater number of pins for the electronic devices of the same size, that is, to decrease the pitch of the leads. For the reason described above, it has been desired to miniaturize the size of the leadframe for semiconductor devices.
In the gull-wing type leadframe, for example, before mounting the leadframe on a printed circuit board, it is necessary to cut the leadframe so as to electrically isolate outer lead sections. As a consequence, new metal surfaces are created at the cut portions of the outer lead sections so that when the leadframe is made of metal that is difficult to solder, there arises a problem that the solder does not perfectly adhere to or cover the new cut surfaces.
Moreover, as the pitch of the leads becomes small, the strength of the leads decreases so that it is necessary to increase the strength of the leads. As a consequence, in recent years, leadframes made of high strength materials having a Vickers hardness (Hv) higher than 250 are practically used. Because in many cases the outer lead sections are designed to have the same width and thickness as those of the leadframe, cutting punches will be broken, when a high strength material is used, thus decreasing the life of the cutting mold. This not only increases the number of times the punches must be ground but also increases the assembling cost. Moreover, as the pitch of the leads becomes fine, the tool utilized for cutting the outer leads becomes fine so that the punches are more liable to be broken, thus shortening the life of the cutting mold.